M61A1/M61A2 20mm Automatic Gun

Click on Picture to enlarge

The Automatic Gun on an A-10

The M61A1 utilized by the F-14 and F/A-18 aircraft is a hydraulically
driven, 6 barreled, rotary action, air cooled, electrically fired weapon, with
selectable rates of fire of either 4000 or 6000 rounds per minute. The M61A2
20mm light weight gun is utilized in the F/A-18 aircraft only. The gun system
is mated to a linkless ammunition storage and handling system. The F-14 has a
capacity of 676 rounds while the F/A-18 has a capacity of 578 rounds of 20mm
linkless M-50 or PGU series electrically primed ammunition. World War II
fighters and bombers were commonly equipped with Browning M2 heavy barrel .50
cal. machine guns which had a maximum firing rate of 1,200 spm. The Gatling
gun had exceeded that rate of fire in 1880. In 1946, U.S. Army Ordnance
Research and Development Service engineers dusted-off the old Gatling
principle and adapted it to create the 6,000 spm M61 series Vulcan 20mm
Gatling gun. The Gatling principle permitted a high rate of fire while
reducing heat and barrel erosion.

In June 1946, the General Electric Company was awarded the contract for
"Project Vulcan". In 1950, GE delivered ten initial model A .60 cal. T45 guns
for evaluation. Thirty-three model C T45 guns were delivered in 1952 in three
calibers: .60 cal., 20mm, and 27mm, for additional testing. After extensive
testing, the T171 20mm gun was selected for further development. In 1956 the
T171 20mm gun was standardized by the U.S. Army and U.S. Air Force as the M61
20mm Vulcan aircraft gun.

The M61 20mm Vulcan is an externally powered, six-barrel, rotary-fire gun
having a rate of fire of up to 7200 spm. The firing rate is selectible at
4,000 spm or 6,000 spm. The gun fires standard electrically primed 20mm
ammunition. The M61A1 is hydraulically or ram-air driven, electrically
controlled, and uses a link less ammunition feed system.

Each of the gun's six barrels fires only once during each revolution of the
barrel cluster. The six rotating barrels contribute to long weapon life by
minimizing barrel erosion and heat generation. The gun's rate of fire,
essentially 100 rounds per second, gives the pilot a shot density that will
enable a "kill" when fired in one-second bursts.

The M61 20mm cannon is a proven gun, having been the US military's close-in
weapon of choice dating back to the 1950s. The F-104, F-105, later models of
the F-106, F-111, F-4, B-58, all used the M61, as does the Air Force's F-15 ,
F-16 and F-22, and the Navy's F-14 and F/A-18. The internally mounted 20mm
cannon system is common to all versions of the F-15. This system combines the
widely used (F-4, F-16, F-18) M61 cannon with 940 rounds (A through D models)
or 500 rounds (E model) of ammunition. The cannon can be loaded with target
practice, armor piercing, or high explosive incendiary rounds. The primary use
of the cannon is in the extremely short range (less than 2000 feet) air-to-air
environment, where more sophisticated air-to-air missiles are ineffective.
Alternately, the cannon has limited usefulness in a ground strafing role.

The M61A2 is a lightweight version of the M61A1. Most of the weight savings
was achieved by machining down the barrel thickness.

Specifications

Designation

M61A1, M61A2

Type

Six-barrel, hydraulically operated 20mm Gatling gun

Contractor

General Dynamics Armament Systems

Rate of Fire

6,000 rounds per minute

Effective Range

Several thousand yards

The
M6120mm "Vulcan"

The 20 mm M61 Vulcan is a
hydraulically or
pneumatically driven, six-barreled,
air-cooled, electrically fired Gatling-style cannon with an extremely high rate of fire. It has been
the principal cannon armament of
United States
military aircraft for five decades.

Development

At the end of
World War II, the
United States Army began to consider new directions for future
military aircraft guns. The higher speeds of jet-engine
fighter aircraft meant that achieving an effective number of hits would
be extremely difficult without a much higher volume of fire. While captured German
designs (principally the
Mauser MG213C) showed the potential of the single-barrel
revolver cannon, the practical rate of fire of such a design was still
limited by ammunition feed and barrel wear concerns. The Army wanted
something better, combining extremely high rate of fire with exceptional
reliability.

In response to this requirement,
General Electric Armament Division resurrected an old idea: the
multi-barrel
Gatling gun. The original Gatling gun had fallen out of favor because of
the need for an external power source to rotate the barrel assembly, but the
new generation of turbojet-powered fighters offered sufficient
electrical power to operate the gun, and electric operation offered
superior reliability to a gas
operated weapon. With multiple barrels, the rate of fire per barrel
could be lower than a single-barrel revolver cannon while still giving a
superior total rate of fire.

The Army issued GE the contract in 1946 for "Project Vulcan," a
six-barrel weapon capable of firing 6,000 rounds per minute. Although
European designers were moving towards heavier 30 mm weapons for better
hitting power, the U.S. chose
20 mm ammunition, trading projectile weight for rate of fire and
muzzle velocity. The first GE prototypes of the T-171 were
ground-fired in 1949.

The development of the F-104 revealed that the Vulcan (later re-designated M61) suffered problems with its
linked ammunition, being prone to mis-feed and presenting a
foreign-object damage (FOD)
hazard with discarded links. A
link-less feed system was developed for the upgraded M61A1, which
subsequently became the standard cannon armament of U.S. fighters. It is
likely to remain in service for at least another decade.

General Electric sold design and production for the M61 (along with
their other rotary cannon) to
General Dynamics.[1]

Description

The Vulcan is a
Gatling gun: each of the cannon's six barrels fires once in turn during
each revolution of the barrel cluster. The multiple barrels provide both a
very high rate of fire--around 100 rounds per second--and contribute to long
weapon life by minimizing barrel erosion and heat generation. Mean time
between jams or failures is in excess of 10,000 rounds, making it an
extremely reliable weapon. The success of the Vulcan Project and its
subsequent progeny, the very-high-speed Gatling gun, has led to guns of the
same configuration being referred to as
Vulcan Cannon, which can sometimes confuse nomenclature on the
subject.

Most aircraft versions of the M61 are hydraulically driven and
electrically primed. The gun rotor, barrel assembly and ammunition feed
system are rotated by a hydraulic drive motor through a system of flexible
drive shafts. The round is fired by an electric priming system where an
electrical current from a firing lead passes through the firing pin to the
primer as each round is rotated into the firing position. The self-powered
version, the GAU-4 (called M130 in Army service), is
gas-operated, tapping gun gas from three of the six barrels to operate
the mechanism. The self-powered Vulcan weighs about 10 lb (4.5 kg) more than
its electric counterpart, but requires no external power source to operate.

Click on Picture to enlarge

M61 ammo belt

The initial M61 used linked, belted ammunition, but the ejection
of spent links created considerable (and ultimately insuperable) problems.
The original weapon was soon replaced by the M61A1, with a linkless
feed system. Depending on the application, the feed system can be either
single-ended (ejecting spent cases and unfired rounds) or double-ended
(returning casings back to the magazine). A disadvantage of the M61 is that
the bulk of the weapon, its feed system, and ammunition drum makes it
difficult to fit it into a densely packed airframe. The feed system must be
custom-designed for each application, adding 300-400 lb (140-190 kg) to the
complete weapon. Most aircraft installations are double-ended, because the
ejection of empty cartridges can cause a foreign-object damage (FOD) hazard
for jet engines and because the retention of spent cases assists in
maintaining the center of gravity of the aircraft. The first aircraft to
carry the M61A1 was the D model of the
F-104, starting in 1959.

A lighter version of the Vulcan developed for use on the F-22
Raptor, the M61A2, is mechanically the same as the M61A1, but
with thinner barrels to reduce overall mass to 202 lb (91.6 kg). The rotor
and housing have also been modified to remove any piece of metal not
absolutely needed for operation and replaces some metal components with
lighter weight materials. The F/A-18E/F
also uses this version.[2]

The Vulcan's rate of fire is typically 6,000 rounds per minute, although
some versions (such as that of the
AMX and the
F-106 Delta Dart) are limited to a lower rate, and others have a
selectable rate of fire of either 4,000 or 6,000 rounds per minute. The
M61A2's lighter barrels allow a somewhat higher rate of fire up to 6,600
rounds per minute.

Until the late 1980s the M61 primarily used the M50 series of
ammunition in various types, typically firing a 100 gram (3.5 oz) projectile
at a
muzzle velocity of about 3,380 ft/s (1,035 m/s). A variety of
Armor-Piercing Incendiary (API), High Explosive Incendiary (HEI), and
training rounds are available. Around
1988 a new round was introduced, the PGU-28, which is now
standard for
US Navy and
USAF aircraft. The PGU-28 is a "low-drag" round designed to increase
muzzle velocity, which rises to 3,450 ft/s (1,050 m/s). It is a SAPHEI (semi
armor-piercing high-explosive incendiary) round, providing substantial
improvements in range, accuracy, and power over the preceding M-56A3 HEI
round. The PGU-28 has not been without problems, however. A
2000 USAF safety report noted 24 premature detonation mishaps (causing
serious damage in many cases) in 12 years, compared to only two such mishaps
in the entire recorded history of the M56 round. The report estimated that
the current PGU-28/B had a potential failure rate 80 times higher than USAF
standards permit.

Disadvantages

Despite its reliability and tremendous rate of fire, the Vulcan has been
increasingly criticized in recent years for its limited performance.

The ballistic characteristics of the 20 mm round are relatively poor,
with the projectile losing energy quickly, and its destructive power and
accuracy are lacking compared to the heavier 25-30 mm rounds favored by
European and Russian air forces. Efforts to develop a larger-caliber
replacement for the M61 have thus far had limited success. The USAF spent a
great deal of money in 1970s on the 25 mm
GAU-7 cannon for the F-15
Eagle, using
caseless ammunition, but it proved to be a failure and was abandoned in
favor of the Vulcan. The five-barrel
GAU-12 Equalizer 25 mm gun used in the AV-8B Harrier
II is a Vulcan derivative, but despite greater hitting power (since it fires
a heavier round at virtually the same muzzle velocity) it had yet to find
wide application (except for the
F-35 Lightning-II which was reduced to four barrel).

Another criticism is that despite its high rate of fire, the Gatling-type
weapon is hampered by the time it takes for the weapon to spin up to its
maximum rotation speed (about 0.5 seconds). As a result, a one-second burst
only fires about 70-75 rounds, which some experts feel is not enough of an
advantage over
revolver cannons like the ADEN/DEFA
30 mm weapons to justify the additional weight and complexity. To overcome
this shortfall, the M61A2, with its lower inertia can be powered by a
hydraulic motor with 5,000
P.S.I. (34 MPa)
of hydro power instead of the 3,000 lbf/inē (21 MPa) previously used on the F-15
Eagle and
F-16 Fighting Falcon.

Applications

The Vulcan was first used in the
F-104 Starfighter. The gun was also used in the
F-105 Thunderchief in combat against Soviet-designed MiG fighters over
Vietnam. It was designed into Air Force versions of the
A-7D where it replaced the Naval standard dual cannon, and subsequently
adopted by the Navy on future fighters. Most significantly, it was designed
into an air superiority version of the
F-4E Phantom II which lacked a cannon in all previous versions since it
was believed that beyond-visual-range
AIM-7 Sparrow missiles made guns obsolete. Combat experience in Vietnam
showed that a gun could be more effective than guided missiles in many
combat situations, and that a gun pod was less satisfactory than an internal
gun.

The Vulcan was later fitted into the weapons bay of some
F-106 Delta Dart models and the
F-111 Aardvark. It was also adopted as standard in the teen-series air
superiority fighters, the F-15
Eagle, the F-14
Tomcat,
F-16 Fighting Falcon and
F/A-18 Hornet. Other aircraft include the Italian/Brazilian
AMX International AMX, and the F-22
Raptor. It was fitted in a side-firing installation on the
AC-119 and some marks of the
AC-130
gunships, and was used in the tail turrets of the
Convair B-58 Hustler and
Boeing B-52H Stratofortress bombers.

Two gun
pod versions, the SUU-16/A (Army M12) and improved
SUU-23/A (Army M25), were developed in the 1960s, often used on
gunless versions of the F-4. The SUU-16/A uses the electric M61A1 with a
ram-air turbine to power the motor. This proved to cause serious
aerodynamic drag at higher speeds, while speeds under 400 mph (644 km/h)
did not provide enough air flow for maximum rate of fire. The subsequent
SUU-23/A uses the GAU-4/A self-powered Vulcan, with an electric
inertia starter to bring it up to speed. Both pods ejected empty casings and
unfired rounds rather than retaining them. Both pods contained 1,200 rounds
of ammunition, with a loaded weight of 1,615 lb (733 kg) and 1,720 lb (780
kg) respectively. Unfortunately, during service in the
Vietnam War the pods proved to be relatively inaccurate: the pylon
mounting was not rigid enough to prevent deflection when firing, and
repeated use would misalign the pod on its pylon, making matters worse.

Main article:
Survey of U.S. Aircraft Gun Pods

A variant with much shorter barrels, designated the M195 was also
developed for use on the
M35 Armament Subsystem for use on the AH-1G
Cobra helicopter. This variant fed from ammunition boxes fitted to the
landing skid and was developed to provide the AH-1 helicopter with a longer
ranging suppressive fire system before the adoption of the
M97 Universal Turret mounting the
M197 cannon.

The M61 is also the basis of the US Navy
Mk 15 Phalanx CIWS system and
the
M163 VADS Vulcan Air Defense System (the M168 variant is
used). Both are considered inadequate for current missile and aircraft
threats, and are being replaced by
surface-to-air missile (SAM) systems like the
FIM-92 Stinger and
RIM-116 Rolling Airframe Missile.

Specifications (M61A1)

Type: six-barrel
rotary cannon

Caliber: 20 mm (0.787 in)

Operation:
hydraulically operated, electrically fired

Length: 73.8 in (1.88 m)

Weight (excluding feed system): 248 lb (112 kg)

Rate of fire: 6,000 rounds per minute (lab terms)

Muzzle velocity: 3,450 ft/s (1,050 m/s) (with PGU-28/B round)

Projectile weight: (HEI) 3.5 oz (100 g)

M61 A1 20mm "Vulcan"

Introduction

The General Electric M61A1 Vulcan is a 6-barrel 20mm cannon of the gatling-type.
It fires standard M50 ammunition at 6,000 rounds per minute (rate selectable
in certain installations).

History

In 1947 the brand new USAF made a request for a new aircraft gun. The
lesson of WWII was that the German, Italian and Japanese fighters could reach
out and touch the Americans with their cannon main armament, while the latter
had to get up close and personal with the .50 cal main armament of the P-51
and P-47. The 20mm Hispano carried by the P-38 was a relatively low velocity
weapon. The idea of hooking up unlimited ammo to a late 19th century "Gatling"
type weapon, and powering it electrically was attempted by the US
Navy for use on their Torpedo/gun boats. Results were high rate of fire
and a high rate of barrel wear out. With propellants of the time and period
metallurgy it was a good idea but one that was not ready until new technology
came along.

In 1950, the US General Electric Company began designing a cannon for USAF
fighters under the Vulcan Project, based on the multi-barrel concept pioneered
by Richard J. Gatling in the 19th century. The Vulcan was first fired in
pre-production form in 1953, and made its first flight in the Lockheed F-104.
Initial problems with the gas bleed resulted in a temporary suspension of
firing tests, until a better venting system for the F-104 gun compartment was
designed.

Today, after a period of neglect (when guns were thought to be rendered
obsolete by missiles), the M61 in one form or another, is an integral part of
the armament of modern fighters such as the F-15, F-18 and of course the F-16.

Recently, GE's armament
division was acquired by Martin Marietta, so the M-61 is now officially known
as the Lockheed-Martin M61A1.

Construction

The multiple-barrels cannon offers both advantages in firing rate and in
barrel life. As the 6 barrels revolve, they proceed through the different
stages of the gun firing cycle. Each barrel is fired as it passes through the
top position, after which the spent case is extracted and ejected, and a new
round is fed and chambered - all at different positions on the circle
described by the revolving barrel. This means that the firing rate is
multiplied by the number of barrels, as six barrels are being loaded in
parallel. Furthermore, since each barrel is only fired at 1/6th of the total
firing rate, barrel wear and tear is not increased. Major drawback is that
ammunition is consumed at a vast rate, requiring large ammo magazines.

Click on Picture to enlarge

Gun Maintenance at Hill AFB: the barrel assembly has already been removed,
while the ammo drum is currently being lifted out. (F-16.net photo)

Click on Picture to

The Gatling gun of an A-10 Thunderbolt II at Osan Air Base, Korea.

The gun is fed through long linked belts of ammo, and although the gun
appeared from early on in the development to be extremely reliable, the
unprecedented rate of fire caused severe problems with these belts. The links
connection the rounds often bent, broke or stretched, causing the gun to jam.
Furthermore, provisions had to be made to dispose of the links. As a result,
development of a new linkless feed system quickly started. Inside the drum,
the rounds (tips to the middle) are placed in a giant Archimedean screw which
moves them into the conveyor belt feeding the gun. In the F-16 and some other
installations (M61A1 installations are tailor-made to each aircraft type), the
empty case is transported back to the drum via a second conveyor belt. Both
conveyor belts are housed in strong flexible ducts, and are powered by the
gun, as well as the screw inside the drum which is driven via a high-power
flexible coupling.

Most members of the M61A1 family are driven by the aircraft's hydraulic
system, or (exceptionally) by the electrical power system. 35hp is needed to
drive the gun at full firing rate, and barrels rotate anticlockwise when
viewed in the direction of firing. The stationary breech housing has a deep
elliptical slot in its inner wall in which run 6 cam followers, on the breech
of each barrel. The followers are driven linearly in and out of the associated
breech, successively chambering, firing and extracting the rounds. The breech
rotor, to which are attached the 6 barrels, revolves inside the breech
housing.

At the muzzle end, the 6 barrels are fitted in a clamp. This clamp can be
replaced by other models, thereby offering a means to vary the barrel angles
and create slightly different dispersion patterns. The inside of each barrel
is provided with a twisted groove to give the rounds a spinning motion.

F-16 Installation

The M61 installation in the F-16 encountered some initial problems, most
notably in September 1979 when firing the gun was temporarily forbidden. Two
incidents had occurred in which gun firing resulted in un-commanded yawing
movements. The cause of this problem was an accelerometer in the
flight-control system being affected by the vibrations caused by the operation
of the gun. The accelerometer fed false data into the flight computers, which
initiated the yaw movements. Simple insulating the accelerometer from
vibration solved the problem. All 106 operational F-16s delivered to date were
retrofitted during 1980.

The ammo drum is located just aft of the cockpit, with the ammo loading
access door in the bottom half of the starboard wing, next to the air intake.
The gun itself is located in the upper port side of the fuselage, with the gun
port on the port side of the cockpit. The ammo drum has a 511-round capacity.

The gun controller is the electronics unit which actually controls the
firing of the gun. A voltage pulse is sent out from the gun controller to fire
each round in a firing burst. At the end of a burst when the trigger is
released, the gun clears itself. In the clearing operation, 5 to 9 unfired
rounds are cycled through the gun without firing pulses, and are fed back to
the ammo drum. These rounds are carried for the duration of the flight as
spent rounds and cannot be used. The SMS (Stores Management System) has a
rounds remaining counting function which counts each firing pulse from the gun
controller and subtracts these from the loaded number of rounds. In the
clearing operation, however, there are no pulses or any way of determining the
actual number of rounds cleared, therefore the SMS assumes 7. Due to this fact
there can be a discrepancy between the rounds remaining on the SCP and the
actual number of rounds left to be fired. This discrepancy can become larger
with increasing number of clearings.

Operational Use

Click on Picture to enlarge

A close-up of the gunport of a Dutch F-16B after a live-firing exercise.
Hot gasses from the muzzle have left carbon traces, which -if not removed-
corode the paint.

Close up of the M61 gunport on a Norwegian F-16A. The copper color is
copper-grease, applied to the gunport to protect it from the hot gasses
during firing. Burns from the gasses are difficult to clean after a firing
period.

The gun takes about 0.3 seconds to wind up to the full rate of fire, and
half a second to wind down again. Some critics (notably defense analyst Pierre
Sprey in a paper called First Rounds Count) argue that the aim is at it
truest when the pilot pulls the trigger, and that it starts wandering off
almost immediately.

Therefore, the 0.3 seconds delay would cause the gun to fire just after
the piper was best aligned with the target. A revolver cannon such as the
Mauser BK27 (fitted in the Tornado) does not have this problem, as it reaches
its maximum rate of fire instantly. A simple calculation however shows that
the M61 fires 70 rounds in the first second (6,000 rounds/minute = 100/second.
30% of the first second is waisted on winding up, so that leaves 70 rounds
fired in the first second). The BK27 fires at 1,700 rpm, or 28 rounds per
second.

In order to fire the same amount of rounds in the first second, you'd still
need 3 BK27's in stead of one Vulcan - quite a challenge to fit them in the
limited space available in an F-16!

The rate of fire of 6,000 rpm or 100 rps means that shells are spaced by
0.01 sec. A MiG-29, with a length of 56ft 5in (17.20m) and 90? angle-off (i.e.
with a direction of flight perpendicular to the direction of flight of the
F-16), flying at 543kts (1,000 km/h) or 278m/sec travels 2.78m in 0.01 sec.
Therefore, the Fulcrum will be hit at least 5-6 times if the aim is true
(17.2/2.78=6.187).

Sound

Due to the extremely high rate of fire of the Vulcan, it is impossible to
distinguish between individual 'shots'. Quite different from the typical movie
sound-effects, the M61 sounds more like a heavy concrete drill. The 5 sec.
sound fragment is
actually from a VADS (Vulcan Air Defense System), used by a.o. the Belgian Air
Force for point defense of its airfields. Unfortunately, only the .AU file
format is currently available - most browser however support this format.

Ammunition

Click on Picture to enlarge

The components that make up a complete round are a brass cartridge case, an
electric primer, propellant powder, and the projectile. The projectile is fired
when an electrical pulse is applied to the primer. The resulting flame passes
through a gas vent leading to the propellant chamber and ignites the
propellant. As the propellant burns, it forms a gas which forces the
projectile through the gun barrel. The only significant difference between the
five types of ammunition is in the projectile. Located at the rear of all
projectiles is a band of soft metal that seats in the grooves of the gun
barrel. The grooves in the barrel are twisted so that the projectile receives
a rotating motion as it travels through and leaves the gun barrel. This
rotation is induced to provide stability in flight. The soft band also serves
to prevent the propelling gas from escaping past the projectile.

Dummy Ammunition

The dummy ammunition color code may be either bronze or shades of gray or
tan. The case will be steel or plastic. Dummy ammunition is used to check out
the gun system.

Target Practice Rounds

M55A1/A2 Target Practice Round (M220 TP Tracer Round)

The M55A1 and M55A2 target practice (TP) round is ball ammunition, with a
body made of steel. The projectile is hollow and does not contain a filler.

Armor-Piercing Incendiary Round

The body of the M53 armor-piercing incendiary (API) projectile is composed
of solid steel. The nose of the projectile is made of aluminum alloy, charged
with an incendiary composition, and sealed with a closure disk. The projectile
does not require a fuze because it ignites upon impact.

High Explosive Incendiary Round

The M56 high-explosive incendiary (HEI) round contains an HEI projectile.
The round is used against aircraft and light targets. The projectile explodes
with an incendiary effect after penetrating the surface of the target. HEI
projectiles require a fuze which has a delay arming distance of 20 to 35 feet
from the muzzle of the gun. Centrifugal force, created by the projectile spin,
allows the detonator to align with the firing pin and the booster, thereby
arming the round. Upon impact, the projectile presses into its target,
crushing the nose of the fuze and forcing the firing pin against the
detonator. The booster, initiated by the detonator, causes the projectile to
explode.

PGU-28

Starting with
block 50 (as far as the F-16 is concerned anyway), provisions have been
made to fire the new 'hotter, faster, farther' PGU-28 round. It reputedly
travels three times as far as the standard M53 round, effectively closing the
gap between the Sidewinder minimum engagement range and the gun's maximum engagement
range.

World War II fighters and bombers were commonly equipped with Browning M2
heavy barrel .50 cal. machine guns which had a maximum firing rate of 1,200
spm. The Gatling gun had exceeded that rate of fire in 1880.

In 1946, U.S. Army Ordnance Research and Development Service engineers
dusted-off the old Gatling principle and adapted it to create the 6,000 spm
M61 series Vulcan 20mm Gatling gun that now arms many of our U.S. Air Force
and U.S. Navy supersonic fighters, including the F-14, F-15, F-16, F/A-18 and
the F-111A. The Gatling principle permitted a high rate of fire while reducing
heat and barrel erosion.

In June 1946, the General Electric Company was awarded the contract for
"Project Vulcan". In 1950, GE delivered ten initial model A .60 cal. T45 guns
for evaluation. Thirty-three model C T45 guns were delivered in 1952 in three
calibers: .60 cal., 20mm, and 27mm, for additional testing. After extensive
testing, the T171 20mm gun was selected for further development. In 1956 the
T171 20mm gun was standardized by the U.S. Army and U.S. Air Force as the M61
20mm Vulcan aircraft gun. The M61 20mm Vulcan is an externally powered,
six-barrel, rotary-fire gun having a rate of fire of up to 7200 spm. The
firing rate is selectible at 4,000 spm or 6,000 spm. The gun fires standard
electrically primed 20mm ammunition. The M61A1 is hydraulically or ram-air
driven, electrically controlled, and uses a linkless ammunition feed system.

A later version of the earlier M-61, the M-61A1 is used on many
fighters. Due to spin up time of 0.3 to rate, the first 0.5 s only 20 rounds
are fired. There are also sources which say it fires 47 rounds the first
second, for a throw weight of 4.6 kg. If this is correct, it might refer to
time from press of trigger, and the numbers 72 (or 75) from the time the first
round is fired. Or perhaps it refers to the M-61.
When the barrels are rotating slowly during spin up and spin down, a few
rounds (around 15 in total) are expended, but not fired as the barrels aren't
clear of the aircraft structure.
A lightweight, 93 kg, version with faster acceleration is being made for
F/A-18, F-22 and Taiwan's IDF.

The old new Gatling: The M134 "Minigun"

M61 Vulcan, GAU-8/A Avenger and others.

Click on Picture to enlarge

Way
back, in 1862, someone dr. Gatling of USA patented a manually operated,
multi-barreled mechanical "machine gun", one of the most successful of such
designs. Being gradually updated, this weapon, in many versions, served with
number of armies around the world as an infantry support or a light artillery
weapon. Usually chambered for the contemporary general issue rifle cartridge,
the XIX century Gatling guns usually have 6 to 10 barrels, mounted around the
single axis. Some naval Gatlings, thought, have had calibers up to 1 inch
(25mm), and some derivatives, like the Hotchkiss, were up to 53mm in caliber.
When crank at the back of the gun was rotated, the "bundle" of barrels rotated
too. Each barrel had its own bolt, which completed full reloading circle per
one turn of the barrels bundle. This resulted in the rate of fire as high as
200+ rounds per minute - quite an achievement in the era of the single-shot
and even magazine fed rifles. While first successful "true" machine guns
appeared by 1890, some Gatling guns seen use until World War 1, especially
with the navies.

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As the automatically operated machine guns became more and more mature,
Gatling guns were gradually forgotten, until the late 1940s. At that time, the
speed of combat aircrafts became so high, so even most fast-firing
conventional machine guns became too slow to achieve desirable number of hits
during the very brief encounters. This spawned the famous "Project Vulcan",
that was intended to develop a super-rapid firing weapon for US Air Forces.
Project was handled by the General Electric Co. First tests were conducted
with the late 19th century Gatlings, fitted with electrical drive instead of
manually operated crank; this immediately resulted in the rate of fire of
about 4 000 rounds per minute, which was very impressive (it must be noted,
that such tests were first conducted in early 1890s, but lead to no practical
results at that time - there were no need in the rate of fire of up to 3 000
RpM). Further development resulted in some experimental, electrically driven,
.60 caliber machine guns with 6 barrels, and, in 1956, the 6-barreled 20mm
T171 gun was officially adopted as the M61 aircraft gun. This gun could fire
at the rate of 4 to 6 thousands rounds per minute. This achievement is
possible due to the fact that gun has multiple barrels, and the rate of fire
per one barrel is about 1 000 rounds per minute or even less, allowing them to
not to overheat. M61 became the main aircraft gun for US AF, and also was used
on M161 and M163 Vulcan ground anti-aircraft gun mounts. The navies also
turned back to Gatlings with the Vulcan-Phalanx CIWS (Close-In Weapon System).

Click on Picture to enlarge

When
USA entered the Indo-China troubles during early 1960s, they soon found
that they need to arm their helicopters, to provide additional firepower
against enemy infantry. These applications also required a lot of
firepower delivered in the short periods of time, so General Electric
designers simply scaled the M61 gun down, for 7.62x51mm NATO ammunition.
The resulting weapon, known as the M134 Minigun, could fire up to 4 000
rounds per minute, and soon found its way to the various helicopter
mounts. It was mounted on chin turrets and in wing pods on AH-1G "Cobra"
attack helicopters, on door, pylon and pod mounts on UH-1 "Huey" transport
helicopters, and on many other helicopters and aircrafts, including famous
"Gunship" airplanes like A/C-47 and A/C-119.

Usually, AH-1G "Cobra" carried one or two Miniguns in its chin turret,
with the 2 or 4 thousands rounds of ammunition. The UH-1 could carry one
or two (or even more) Miniguns on various mountings, with as much as 12
000 rounds of ammo available for "immediate delivery to enemy".

With the introduction of the 5.56mm ammo into the military service,
Americans attempted to scale the the Minigun further down, resulting in
the weapon, known as XM-214 Microgun. This little beast had 6 5.56mm
barrels, was electrically driven and could fire up to 10 000 rounds. But
the 5.56mm was way too weak for air-to-ground or ground-to-air
(anti-aircraft) applications - the key niches of the modern Gatling
systems. For the infantry, the Microgun had almost no use, being to heavy,
complicated and with too much rate of fire and recoil (recoil force was up
to 110 kg / 240 lbs at the full rate of fire). Infantry simply does not
needed an extremely rate of fire to deal with the enemy infantry, and for
AA use (where such high RoF makes sense), the 5.56mm (.223mm) and 7.62mm
(.308) ammo is way too weak.

Click on Picture to enlarge

Others than noted, US also developed a number of other
Gatling-type designs chambered in .50BMG, 20mm, 25mm, and even 30mm (like the
famous GAU-8/A "monster gun", shown at left, mounted on the A10 Warthog
attack planes). It also must be noted that the USA was not the only country to
exploit Gatling ideas. USSR (and latter Russia) built several aircraft and AA
guns and machine guns. For helicopter use, they made a 4-barreled machine guns
in

7.62mm and 12.7mm, and for aircrafts - 23mm and
30mm 6-barreled guns. Some 6-barreled 30mm Gatling type guns also used by
Soviet and Russian navy for shipborne AA installations, sometimes coupled with
short-range AA missiles.

Modern Gatling guns - pros and cons.

Key advantage of the modern, externally powered Gatling type guns, is the
extremely high rate of fire, usually 4 to 6 thousands of rounds per minute
(RPM), sometimes up to 10-12 thousands RPM. This rate of fire is necessary to
deal with the fast-moving targets, when the engagement time is very short.
Such targets are mostly aircrafts, or ground targets, fired at from aircrafts.
The downside of multi-barreled systems is they relative complexity, heavy
weight, and requirements for external power (electrical, pressured air or
hydraulics). There are few self-powered (gas-operated) Gatling type guns, but
they still are much bulkier and heavier, than the conventional single-barreled
guns. Another drawback of the Gatling-type guns, which is essential for aerial
combat, is that the gun requires some time to get on to the full speed (rate
of fire) after the trigger is pressed. For the M61 Vulcan cannon, for example,
the "speed up" time is about 0.4 second or so.